There is a good deal of confusion amongst authors over the exact identity of this person. He has been allocated as a son of William of Gellone and his second wife Guitbergis (or Vuithbergis) on the basis of the Liber Manualis of Dhuoda, wife of Bernard of Septimania, one of William's sons by his first wife. Otherwise, he has been recently hypothesised as the son or grandson of Adalard, Count of Chalon, who defended that site against Waifer of Aquitaine.[1] Warin was thus Count of Chalon by heredity or by grateful gift of the king[2] on account of his father's service. This latter theory hinges on the assumption that there were two Guerins who have been subsequently confused: Guerin I and his son Guerin II. Neither descent from William of Gellone nor the two persons hypothesis are universally accepted. His proximity, in extant documents of the time, to Bernard of Septimania has been used as evidence for a relationship to that family, as has the existence of a related "Count Guerin" in later charters of the 850s and 860s. Guerin has been suggested as a brother of Bernard I of Auvergne, whose relationships are unknown.[3]

Charles sent Guerin to expel Bernard of Septimania from Toulouse in 842 and then against Gothia in 843. After the Treaty of Verdun in August that year, he was the dux and marchio in Provence under Lothair.[3] He may have inherited that office from Leibulf around 829.[5] In 844, he received Autun, which had been stripped from Bernard's heir William. The French historian Pierre Andoque asserts that Bernard was captured in 843 by Guerin in Uzès and brought before Charles to be executed in 844. He was succeeded in 845 by Fulcrad as duke, with Marseille going to a count Adalbert.